HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon in the status bar when browsing.

The extension can validate the HTML sent by the server or the HTML in the memory (after Ajax execution). The extension is based on Tidy and OpenSP (SGML Parser). Both algorithms were originally developed by the Web Consortium W3C. And now extended and improved by a lot of persons. Both algorithms are embedded inside Mozilla/Firefox and makes the validation locally on your machine, without sending HTML to a third party server.

HTML Validator finds HTML errors and classifies them in 3 categories:

• errors: HTML errors that Tidy cannot fix or understand.
• warnings: HTML errors that Tidy can fix automatically
• (optional) accessibility warnings: HTML warnings for the 3 priority levels defined in W3c WAI

Download HTML Validator Firefox Addon